1. Field of the Invention
The invention relates to a method of correcting picture steadiness errors in telecine scanning, in which method light modulated by film frames is converted into a video signal and in which, with reference to the frame position, the picture component in the video signal is corrected by means of a correction signal. The invention also relates to an arrangement for performing the method.
A film scanner converts the optical frames stored on a film into a video signal. To obtain a video signal with picture contents which are free from steadiness errors, each film frame should be mechanically positioned in exactly the same way during the scanning process and the illumination process. The single film frames are usually positioned by means of a pin registration, in which the position of sprocket holes at the edges of the film is detected and evaluated. However, due to mechanical inaccuracies, the pin registration yields imperfect results so that more or less disturbing horizontal, vertical, translatory and/or rotary picture steadiness errors are visible in the displayed video image. Some of these picture steadiness errors are based on reasons of principle, for example, those picture steadiness errors which are due to damage of the sprocket holes, irregularities of the film, splices in the film, fluctuations of the film feed speed, or film abrasion in the sprocket holes.
2. Description of the Related Art
To eliminate picture steadiness errors in telecine scanners based on the flying spot principle, it has been proposed (U.S. Pat. No. 4,104,680) to optically scan the edges of the sprocket holes in the horizontal direction and to derive a correction signal from the generated edge signal. The correction signal shifts the horizontal deflection of the scanning beam of a scanning tube projecting a TV frame on the film to be scanned. However, this correction method cannot be used in film scanners with CCD line sensors scanning the film. A further drawback of the optical sprocket hole scanning is that, even when the exact spatial position of the sprocket holes is determined, the position of the individual film frames cannot be determined with the desired accuracy of approximately +/-0.1 times the height of a line, because the standard itself already allows larger positional deviations between the sprocket holes and the film frames. It can be concluded that frame stabilization results can be considered to be unsatisfactory if the signal for correcting picture steadiness errors is derived in accordance with a method in which the position of sprocket holes as a reference for positioning the film frames is used in the telecine scanning process.
U.S. Pat. No. 5,194,958 discloses a method of correcting picture steadiness errors (or film weave correction) in which a correction signal for frame stabilization is derived from L-shaped marks provided on the film in gaps between adjoining frames. The correction signal controls a video signal processor, for example, a digital video-effect apparatus. Since commercially available films are not provided with L-shaped marks, only special films can be corrected for picture steadiness by means of this known method.
United Kingdom Patent GB 2,187,913A also discloses a device for detecting picture steadiness errors, in which a motion vector measuring circuit derives displacement signals from the video signal of the film scanner, these displacement signals representing the horizontal and vertical offset between successive frames. The displacement signals control a 2-dimensional interpolator which interpolates the video signal of the film scanner, while excluding the frames whose motion contents are based on a frame offset, a frame magnification or diminution, or a scene of changes. In the motion vector measurement, distinctions between neighboring pixels within a line, as well as distinctions between neighboring lines and frames are determined. To generate a correction signal representing a steadiness error, the displacement signals derived in the motion vector measuring circuit are subjected to a computation based on a concise Taylor expansion. The drawback of the known device is that it is not suitable for stabilizing the frame position to a sufficiently accurate extent. However, a particular drawback is that a correction signal is prevented from being generated when there is a multitude of given picture contents.